1. Field of the Invention
The present invention relates to an image forming apparatus designed to offer a capability to form an image of an original quickly and accurately based on its image data.
2. Description of the Related Art
The tendency in recent years has been to implement multi-color features in copying machines and printers. Image forming apparatuses featuring both multi-color and monochrome (black-and-white) image forming functions in a single unit are in widespread use today. An image forming apparatus featuring the monochrome and multi-color functions includes an image processing unit which produces a digital image signal from a scanned original image through a photoelectric conversion process. The image forming apparatus of this kind has image processing capabilities to properly cope with a wide variety of originals which include a full-color original, a unicolor original, an original carrying characters (printed text) and line art, a photographic original (printed on photographic printing paper), printed material (an original containing a halftone dot pattern) and an original containing halftone areas and line art, such as a map.
Conventionally known image forming apparatuses usually have keys on an operator panel to enable an operator to specify types of originals so that the apparatus can correctly read various types of original images and produce printed images of high quality. While the apparatus can set appropriate parameters suited for performing image forming operation with each type of original if the operator correctly operates the keys for specifying original types, some technical knowledge is needed for distinguishing the types of individual originals. However, the operator is often unable to distinguish the types of originals and occasionally specifies an incorrect type. It is therefore often impossible to obtain satisfactory printed images.
A previous approach to the resolution of the aforementioned problem is to develop an image forming apparatus provided with an image processing unit which can distinguish types of originals by itself, so that the operator is relieved of the need to distinguish individual original types, as proposed in Japanese Laid-open Patent Publication Nos. 1996-251402 and 1996-251406, for example.
Another previous approach is found in Japanese Laid-open Patent Publication No. 2002-232708 which discloses an image forming apparatus having a capability to automatically distinguish various types of originals including multi-color and monochrome originals with high accuracy by a simple method using image area separating means, thereby avoiding an increase in the scale of circuitry.
Generally, the image forming apparatus featuring both monochrome and multi-color image forming capabilities handles monochrome originals more frequently than multi-color originals, so that an important performance criterion in monochrome image forming is high processing speed. In contrast, great importance placed on the quality of printed images, rather than speed, in multi-color image forming.
When automatic image forming mode is selected in the aforementioned image forming apparatus of the prior art, it prescans an original to determine whether the original is a multi-color original or a monochrome (black-and-white) original. This image forming apparatus automatically prescans not only multi-color originals but also monochrome originals for which high-speed processing is of great performance, requiring a long time before printed images are obtained.
It is often the case that this kind of image forming apparatus is initially placed in the automatic image forming mode by default. If the image forming apparatus is so preset, it will automatically prescan each original, even if it is monochromatic, making it impossible to perform high-speed image processing regardless of the operator's intention.